1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer or a copy machine, which uses a charging device.
2. Description of the Related Art
In an image forming apparatus, such as a printer or a copy machine, a charging device has been widely used which charges a photoreceptor that rotates in one direction with a predetermined voltage V0 by corona discharge. Corona discharge has an advantage in that it uniformly charges the photoreceptor. However, the corona discharge uses a high direct-current voltage in a range of 4 to 6 kV. Therefore, ozone generation is caused at the time of corona discharge and thus affects the environment.
JP-B-3-52058 suggests a contact-type charging device in which a brush or roller comes into contact with a photoreceptor, an alternating current voltage is applied thereon, a desired charging voltage is obtained from a relatively low voltage, and discharge to the ozone is rarely caused.
Further, each of JP-B-41-21432, JP-A-64-35459, JP-A-1-35460, JP-A-4-21875, JP-A-4-30186, Japanese Patent Number 3230019, JP-A-6-289688 and JP-A-2005-331846 discloses charging method, in which a uniform surface voltage is applied to a photoreceptor, an alternating current voltage source is not used, and a capability to remove a voltage history of the photoreceptor is excellent.
Further, JP-A-2001-100469 suggests an image forming apparatus in which high image quality can be obtained even under various environmental conditions.
A charging device that is disclosed in JP-B-3-52058 has the following advantages. In the charging device, discharge to the ozone is great reduced (the charging device discharges the ozone in a range of 1/10 to 1/100 to the ozone when using corona discharge) to realize uniform charging, and a capability to remove the voltage history of a photoreceptor is excellent. As a result, according to the charging device that is disclosed in JP-B-3-52058, a discharging process does not need to be performed before a charging process, and a small-sized image forming apparatus can be achieved. However, the charging device that is disclosed in JP-B-3-52058 has disadvantages in that a size of an alternating current voltage source is increased, and a vibration sound is generated at a nip portion due to an alternating current electric field.
According to the charging methods that is disclosed in each of JP-B-41-21432, JP-A-64-35459, JP-A-1-35460, JP-A-4-21875, JP-A-4-30186, Japanese Patent Number 3230019, JP-A-6-289688 and JP-A-2005-331846, as a primary charger and a secondary static eliminator, a contact-type charger or a discharging device using needle-shaped electrodes are combined, and the primary charger charges a surface of a photoreceptor such that a voltage at the surface of the photoreceptor is increased to a voltage V1 higher than a predetermined voltage V0 and then the secondary static eliminator discharges the voltage at the surface of the photoreceptor such that the voltage at the surface of the photoreceptor becomes the predetermined voltage V0. In an image forming apparatus using the charging device that has the above-described structure, an electrostatic latent image is formed on a photoreceptor charged with the predetermined voltage V0 by an exposure device, a visible image is formed on the electrostatic latent image by a developing device using a toner. Then, a transfer device transfers the visible image on the photoreceptor to paper serving as a transferred material, or an intermediate transfer medium provided between the paper and the photoreceptor. Here, when the visible image formed on the electrostatic latent image is transferred to the intermediate transfer medium, the visible image on the electrostatic latent image is transferred from the intermediate transfer medium to the paper.
Then, the toner image that is transferred to the paper in the above-described method is transported to a fixing device so as to be fixed on the paper. However, at this time, there is a toner that is not transferred to the paper through a transfer operation and remains on the photoreceptor. The reason whey the toner remains on the photoreceptor is because the toner is charged with a polarity opposite to a predetermined polarity due to discharge at the time of the transfer operation. Then, the toner that remains on the photoreceptor is removed from the photoreceptor by a cleaning blade.
However, small-diameter components externally added to the residual toner, such as, for example, silica, are not completely removed from the photoreceptor by the cleaning blade, and pass through the cleaning blade and reach the charging device. In the charging device having the above-described structure, if a brush-shaped charger is used as a primary charger, when the charging device is a contact-type charging device, Paschen discharge is generated in a minute gap and the surface of the photoreceptor is charged. However, at this time, if small-diameter residual materials remain on the photoreceptor, strong discharge is generated due to local electric field concentration on the basis of the small-diameter residual materials, which causes image irregularities. In particular, according to the phenomenon of the electric field being concentrated on the basis of the small-diameter residual materials, when the humidity is low, the strong discharge may be easily generated, in particular, in a roller-shaped charger having a smooth surface.